JNTU
JNTU B.Tech III POWER ELECTRONICS Previous Question Paper (January/February - 2023 - R13)
Note: i) Question paper consists of Part A, Part B. ii) Part A is compulsory, which carries 25 marks. In Part A, Answer all questions. iii) In Part B, Answer any one question from each unit. Each question carries 10 marks and may have a, b as sub questions.
Part A
What is commutation? What are the two main types of commutation?
Answer
Commutation is the process of turning off a thyristor (SCR) by reducing its forward anode current below the holding current level. The two main types of commutation are natural commutation, which occurs when the supply voltage naturally reverses polarity, and forced commutation, where external circuitry is used to force the anode current to zero in DC circuits.
What are the conditions which must be satisfied for turning on time SCR with a gate signal?
Answer
For an SCR to be successfully turned on using a gate signal, the following conditions must be satisfied:
- The anode must be at a positive potential with respect to the cathode, placing the device in forward blocking mode.
- The gate signal applied must be of sufficient magnitude (gate current) and duration to trigger the internal regenerative action of the SCR.
- The gate current must be applied while the anode-to-cathode voltage is below the forward breakover voltage, ensuring control via the gate terminal rather than spontaneous breakdown.
What are the performance factors of line commutated converters?
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What are the differences between half controlled bridge and fully controlled bridge rectifier?
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List out the applications of DC chopper.
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What is meant by step down and step up chopper?
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What are the different types of AC voltage controller?
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What are the merits and demerits of three phase bridge cycloconverter?
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What is space vector PWM?
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What is the need for voltage control in an inverter?
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Part B
Describe the different modes of operation of a thyristor with the help of its static V-I characteristics.
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The gate circuit of an SCR has a source voltage of 15 V and the load line has a slope of -120 V per ampere. The minimum gate current to turn on the SCR is 25 mA. Calculate (i) Gate source resistance and (ii) the gate voltage and gate current for an average gate power dissipation of 0.4 watts.
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What is the necessity of connecting SCRs in series and what are the problems associated with series connection of SCRs? How are they eliminated?
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Draw and explain the switching behavior of power MOSFET.
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Explain the effect of freewheeling diode in details. Also, justifies the statement “freewheeling diode improved the power factor of the system”.
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A single phase semi converter is operated from 120 V, 50 Hz AC supply. The load current with an average value Idc is continuous and ripple free firing angle α = π/6. Determine (i) displacement factor, (ii) harmonic factor of input current and (iii) input power factor.
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Describe the operation of a three phase, half controlled bridge converter with associated waveforms.
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For a 3-phase full converter operating from a 3-phase, 415 V, 50 Hz supply, derive an equation for the average output voltage in terms of the overlap angle μ. Also find out the reduction in output voltage.
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With the circuit diagram and output voltage waveforms, explain the principle of operation of a chopper.
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With the help of voltage and current waveforms, explain the working of first quadrant chopper. Give the complete time domain analysis of class A chopper.
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Draw a schematic diagram of a single phase AC chopper and discuss in brief with output voltage and current waveforms.
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A Dc On-OFF chopper operating at 1 kHz and duty cycle of 10 % is supplied from a 200 V source. If the load inductance is 10 mH and resistance 10 ohms, compute the maximum and minimum current in the load.
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Describe the operation of single phase half wave AC voltage regulator with the help of voltage and current wave forms. Also derive the expression for average value of output voltage.
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A single phase voltage controller has input voltage of 230 V, 50 Hz and a load of R = 15 Ω. For 6 cycles on and 4 cycles off, determine (i) rms output voltage, (ii) input p.f., (iii) average and rms thyristor currents.
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Explain the basic principle of working of a single phase to single phase cycloconverter for both continues and discontinues conductions for a bridge type cyclo converter.
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A three phase to six pulse, 50 kVA, 415 V cycloconverter is operating at a firing angle of 45°. And supplying load of 0.8 p.f. Determine input current to the converters.
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Explain the operation of single phase bridge inverter with the help of voltage waveforms.
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Design a series inverter to meet the following specifications: Max. Output frequency = 4 kHz, Load resistance varies from 400 to 100 ohms, Supply voltage = 120 V.
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What is the need for controlling the output at the output terminals of an inverter? Discuss briefly and compare the various methods employed for the control of output voltage of inverters.
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Define pulse width modulation. List the various PWM techniques. How do these differ from each other?
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